Lecture 8: Combustion Flashcards
What is the combustion reaction in words
Fuel + Oxygen —–> Gases + Heat + Light
What is the chemical combustion reaction
CH4 + 2O2 ——> 2H20 + Heat + Light
What are the 3 sources of fuel for combustion
- Solid (woods)
- Liquids (accelerants)
- Gas (propane, butane etc.)
What does heat do in a combustion reaction
Breaks the bonds of the reactants
Why is combustion an exothermic reaction
Releases energy
Why is combustion a continuous reaction
Combustion will continue until the fuel or oxygen runs out
What are the factors affecting the rate of combustion
- Physical state of the fuel (solid, liquid or gas)
- The temperature of the flammable liquids
- What are the different rates of a combustion reaction
- Detonation
- Deflagration
- What is detonation
The speed of combustion is faster than the speed of sound (sonic), Produces a devastating shockwave which destroys everything
*What is deflagration
The speed of the combustion reaction is slower than the speed of sound (subsonic), produces a wave of flame front.
What are the combustible solids
- Pyrolyzable (Woods)
- Non-pyrolyzable (Charcoal, Cigarettes)
What are pyrolyzable solids
Can undergo combustion at high temperatures, without the presence of oxygen.
When wood undergoes pyrolysis what does it produce
gaseous products
Gaseous products + Oxygen =
Flaming fire + heat
How long does combustion of pyrolyzable solids last
Until all solids within the wood have been consumed
What remains following the combustion of pyrolyzable solids
a carbon based residue which undergoes glowing combustion, smoldering
What are non pyrolyzable solids
Undergoes glowing combustion
What is charcoal
Pure carbon
What happens when carbon is heated
Doesn’t break down or produce vapours, instead the surface oxidises and undergoes glowing combustion
How does the oxidation of carbon occur
Oxidation starts at the outer surface, consumes it and moves onto the next surface and continues until there is nothing left.
What are the constituents of inorganic gun shot residue?
-Lead
-Antimony
-Barium
What is required for definitive identification of gun shot residue?
Identification of all 3 elements
*As they can be found individually in legal things
What technique is used to analyse inorganic gunshot residue?
SEM-EDX
What does SEM-EDX stand for?
Scanning electron microscopy - energy dispersive X-Rays
How does SEM-EDX work?
-The microscope produces the image of the particle
-X-ray determines the elemental composition
What happens when a bullet is ejected from a gun barrel?
-Firing pin strikes the primer, detonating it producing flames
-The flames ignite the propellant which produces gas causing the bullet to travel down the barrel of the gun
What are the combustion reactions causing the ejection of a bullet from a gun?
- Firing pin strikes the primer, detonating it
- Fire ignites the propellant
What are the series of combustion reaction causing the ejection of a bullet from a gun called?
A low explosive train
What are the changes in energy causing the ejection of a bullet from a gun?
1.Fire pin strikes primer: Mechanical energy
2.Detonation of primer produces heat: Mechanical to heat energy
3.Heat causes the production of gas: Heat to chemical energy
4.Movement of bullet: Chemical to kinetic energy
What are the 3 factors affecting the acceleration of a bullet?
-Rate of gas production
-Burn rate
-Burn pattern
How does the rate of gas production affect the acceleration of a bullet?
Steady state production of gas is required to ensure smooth movement of the bullet down the barrel
What will happen to the acceleration of the bullet if the rate of gas production is too fast?
The initial momentum won’t be maintained and the bullet will slow down as it exits the gun
What will happen to the acceleration of the bullet if the rate of gas production is too slow?
The bullet will have exited the barrel before all gases have been produced
What is a progressive burn pattern?
burns from end to the other
What is a degressive burn pattern?
Burns from the outside in
What is a neutral burn pattern
Burns in a neutral pattern
produces pores, increasing surface area
Why must burn rate be controlled for the acceleration of a bullet?
To ensure maximum kinetic energy is imparted to the bullet
What shape of propellant particle does a shotgun require?
Disc shape
What shape of propellant particle does a revolver require?
tubular shape
What can propellant particles be coated with that inhibits combustion?
A detterant
What does the detonation speed of an explosive depend on?
The density of the explosive
How does the density of an explosive impact the detonation speed?
Greater density = Faster detonation
What are the shockwaves produced by a bullet?
- Mach cone
- Canneluer
- Wake
What is the shockwave produced at the front of a bullet?
Mach cone
What is the shockwave produced from the middle of the bullet?
Canneleur
What is the shockwave produced from the base of the cone?
Wake
What is a molecular explosive?
-Detonate easily
-Explosives in their purest form
-Difficult to handle safely
-Highly unstable
What is are examples of a molecular explosive?
-Nitroglycerin
-Nitrocellulose
What are low explosives?
-Deflagration explosives
-Won’t detonate in normal conditions
-Will detonate when confined, producing a throwing action
-Decomposed by a flame front moving sub-sonically through the object
What are examples of low explosives?
-Nitrocellulose
-Gun powder/ black powder
What are the uses of low explosives?
-Gun powders
-Flares
-Propellants
What are high explosives?
-Detonating explosives
-Decomposed by high pressure shock waves which move sonically
What are primary explosives?
-A high explosive
-Sensitive to shock and impact
-Detonated by shock, impact, friction etc.
-Produces a powerful shock wave that will detonate 2nd explosives and propellants
What are examples of primary explosives?
-Lead azide
-Lead styphnate
-Mercury fulminate
What are uses of primary explosives?
-Blasting caps
-Military fuses
-Primers in cartridges
What are secondary explosives?
-Main explosives
-Will burn under normal circumstances
-not sensitive to shock or impact
-Only detonates due to primary explosive shock waves
What examples of secondary explosives?
-TNT
-Dynamite (Nitroglycerine)
-HMX
What is the full name for TNT?
2,4,6-Trinitrotolune
What is the full name for HMX?
1,3,5,7-tetranitro,1,3,5,7-tetracycloctane
What are propellants?
-Used to accelerate a projectile to high speed
-Don’t detonate only ignite
What is a single base propellant?
Nitrocellulose
What is a double base propellant?
Nitrocellulose + Nitroglycerin
What is a triple base propellant?
Nitrocellulose + Nitroglycerin + Nitroguanidine
